Automatic trunking system.



E. A. MELLINGBR.

AUTOMATIC TRUNKTNG SYSTEM.

APPLICATION FILED JUNEIO, 1909.

E. A. MELLINGER.

'AUTOMATIC TRUNKING SYSTEM. APPLIGATION FILED 11111310, 1909.

8 SHEETS-SHEET 2.

.Ptentea sept. 8,1914. v`

E. A. MELLINGBR. AUTOMATIC TRUNKING SYSTEM. APPLICATION FILEDJuNB1o.'19n9.

1,1 10,071. Patented sept. 8, 1914.

3 SHEETSMSHEET 3.

UNITED SAT Y @NEIGE EDWARD A. MELLINGER,

0F CHICAGO, ILLINOIS, SSIGNOR 'ro AUTOMATIC aimerais AUTOMATIC TRUNKINGSYSTEM.

To all wiz om t may concern Be it known that I, Enwann A. MELLIN- can, acitizen of the Unit'ed' States of America, and resident of Chicago, Cookcounty, Illinois, have invented a certain new and useful Improvement inAutomatic Trunking Systems, of which the following is a specification. t

My invention relates to telephone exchange trunking systems of thateneral character in which means are provi ed for automatically selectingidle trunks to extend the calls in the direction of the calledsubscribers.

It relates more particularly to systems of that kind in which thesubscribers are divided into groups, a set of trunks provided for eachgroup, andv another set of trunks providcd'whichare common to all of thegroups, whereby each calling subscriberl has access to a set of trunkswhich are individual to his group, and also to a set of trunks which arecommon to the diii'erent groups.

The principal object of my invention is the provision of means wherebyno subscriber can use one ot' the trunks which are common to thediilerent groups until after all of the trunks which are individual tohis group are busv or in use, thus limiting the service normally to thetrunks which are individual to the diiieren't groups, and precluding useof the common or reserved trunks yexcept when the trunks in one or moreof'the different groups are all busy. In this wayv the common trunks aremaintained in reserve. and are kept clear of all tradic except thatwhich may betermed the overload, the normal load or'traiiic being takencare ot' by the trunks that are individual to the diierent groups.Furthermore` and in accordance with my invention, the fact that all ofthe trunks in one group are in use docs not throw the common trunks opento use by the subscribers of all the groups. inasmuch as for thosegroups in which there are traunks still available for .use.`the`common`trunks will Still test busy. so that no calling'subscriber can obtainone of these common trunks unless all thetrunks in his ownindividualgroup are busy. In this wav mv'invention serves to increase theeiiiciencv of. the common trunksgns they are reservedexclusively for theuse lo those subc scribers who are unableV to find idle trunks 1n theirindividual groups', and 'are never accessible to other subscribers.'

Specification of Letters Patent. application :ned :une 1o, 1909 erssubstation Patent wie; serial No. 501,250.

The nature and advantages of my inventlon will, however, hereinaftermore fully appear,

In the accompanying drawings Figure l shows diagrammaticallya callingsubscrib- A and central oiiice switch; ing apparatus involving theprinciples of my invention. T o the line of substation A is allotted theline switch C, which latter is controlled by the master switch D. At Eis diagrammatically represented a first selector switch.

In Fig. Q there is shown diagrammatically a connector switch F, at H isshown a central oiice ringer generator, and at I a busy signalingdevice. To the line of the substation A- there is allotted lthe line.switch C which is like the switch C.

Fig. 3 shows diagrammatically trunk lines of two line switch groups,some of which are individual to one group and some to the other. andpart of which are common tothe two `groups-that is. they may be used bysubscribers in either group when the in- 30 dividual trunks allotted tothat group arelin use.

F l and 2 taken together represent dial gralnniaticallv thc centraloliice switchingr apparat-us involved in establishing a connectionbetween two substations. In Fig. Q-a central battery shown at B'havingits positive terminal preferably grounded at G.. The central oiliceswitching apparatus is ot the `general type disclosed in English. PatentNo. 26.301 of 1906. but with modifi'- cations and circuit changes toadapt them to operate in connection with aVtwo-wire s vstem-that is. asystem in which there is no 'ground connection at the subscriberstelephone. The line switches C and .C and master switch D are modifiedforms of similar apparatus disclosed in T71@ Western. Zrwzz/w'ca/n ofJanuary Q5. 1908..and the i .4m-crimp. Teich/'10710 Journal of rlune (i.100 1908. rPhe first selector E is ofthe general' if" tvpe of selectorswitch shown in the United States Letters Patent No. 815.321. issued scf. Marclrlf'. 1906. to Keith. Erickson &Erif-k` son, while the connectorswitch F is of'th'e general type shown in United States Letters PatentNo.llgl''. issued March '13, 190V. to Keith; Erickson`&v Erickson.

The'xnaster switch D in Fig. 3 controls' the plungers or" one group ofline switches: likewise, the'master switch D control-s a second group ofline switch plaag-ors. To

. common trunks Elli `properlyitafke care of the wheel 10.is secured tothe shaft the first group are allotted exclusively the that they may beused by either group When all oi? its individual trunks a or a are inuse. lt will be seen that each master switch is provided `with a doubleset of bank contacts.`

The second set is so arranged and provided with a guarding potentialthat the master switch Will not .stop opposite any of the unless all ofthe individual trunks of its 4group are in use, under which circumstancethe rotection is removed from the commontru ,and the master switch maystop with the lplungers opposite one of these trunks. Thus itwill beseen that so long as'any oit theindividual trunks of a grouplare idlethe common trunks are not accessihlm The -connnon trunks, are .merelyan'outlet when the calls of a group become so .numerous and ra id that theindividual trunks are ,not su cient to traino.

The subscribers' station, it '.Will be seen, is somewhat different fromthe usual automatic substation, substation. A being arranged andmodified to operate in connection with atWo' Wiresystem. Substation A,for example, .comprises the usual. receiver 2, switch hook 3,..inductioncoil 4f Whichconsists oi the primary Winding 5 and secon: dary windingv6, transmitter` 7, condenser 8, ringer 9, and impulse Wheel 1G Whichhasthe usual impulse 'teeth l1. The" impulse there is also secured the dial(not shown). ll'hen the dial is pulled down the springs 14 and are notseparated, but when it' re.

turns the impulse teeth 11. are adapted to engage the cani 13 upon theend of the spring 14, and carry the latter out .of engagement with thespring l5 once by each tooth. Upon the switch hook central ollit'cswitches in establish u connec-v tion. For example, it .will hepointcdout how the subscrilwr ai substation A may op crate. 'the mainmilice switchcsvto establish connection with substation A (Fig. 2.) thenumber ol1 -`which is i520. lo call the number 22() the subscriber ataube-itation ../Y'., after re- 'into a bank therefore,

12 to whichl 3 there is tlie' f cam i6, which when the switch hook isup,

5; but when the switch' into Midori moving 'the receivertrom the switchhook, operates the calling device for each digit in the Well knowninanner. `When the receiver is removed from the switch hook, and thcsprings 17 and 18 engage, an energizing circuit is closed through thelino relay or' the line switch Cf YVhcn this relay cncrgizes a circuitis closed through the trip or plunger magnet-24, which then operates toattract the plunger ,arm 21 and thrust thc plunger terminal and extendthe calling line conductors to a trunk line leading t0 a `selectorswitch. The dial is operated for ithe first digit, and upon returning tonormal, position operates' the first selector switch to establish'connection with a trunk line lead'- ing to a connector switch. Theimpulses delivered by the dial for the last two digits .are directed tothe connector, which oper iates to place its shaft Wipers in connect-ionwith, the contacts corresponding to the def, .'sired number.' When thesubstation primary circuit is bridged, across .the line conductors 19and20, an energizing circuit is established through the line relay' 23extending from ground G' through the springs Q6' and 27 to the lineconductor 20, through the transmitter 7 ,andprimary Winding impulsesprings 14C and 15, springs 17 and y18 to'thc line conductor 19, springs2 8l and 29, Winding of the vlineaela-y2d, springs 30 and 31 to the bat-'terylead 25, thence igirdund Gr.' .When the :plunger enters the Abankterminal ,andfcloseszthehank springs 32 Eend- B3 in contact, anenergizing ,circuit is closed through the 'motor niagnetrelay 34, whichin` turn. operates'toclose a circuit #through the motor'magnet 35 ofthemaster sw'itel'yD. The motor magnet then operates to advance all idle.plungersl that are iii locked .engagement with the plunger shaftoppositethe next 'idle .bank terminal similar to the one just engaged..The-,circuit through the motor magnet relay extends from ground l l2through the relay 3Q to the common seg nient 3G of the master switchbank Wiper 37,. individual 'segment 38, thr0ugh'- the bank ,springs 32and 33to the battery le'ad 2,

through' battery. B to thence through battery B and .t0-ground Gr; i

The relay-3d, uponenergizing, operates to place the springs 39 and@in'contact,.w.here.

by 'a circuit is .closed through the lmotor,

magnet This 5 circuit jextends from ground G5 through the motor magnet35, 1n-

. .terrupter springs 41 and 42, .motor magnet -relay springs 39 and 4:0to the battery lead Q5, thence through 4battery B and to ground Gt. l

` The motor magnet, upon energizing, attracts itsarmatilre-` 43, and theawl '44 engagesv a` tooth .of the ratchet Wiieel 45 and mores'the latterone-step. W'hen this occurs the spring 46 engages the spring L17(bearing in niind thatthe spring e6, prior to this operation, engages anotch ofthe toothed wheel 48), and when the ratchet wheel 45 -i's givenone step the spring46 slides oft from the top of the notch and engagesthe spring 47. The engagement ofwthese two springs provides a newenergizing` circuitior the motor magnet 85 extending through the springs46 and 47 to the battery lead-25instead of through the lsprings 39 and40. lVhen thecir'cuit is closed for the motorv magnet 35 an energizingcircuits-also 'pro-V vided for the-slow acting relay-:'29s: lThis relaythen operates to separatethe springs 30L and 3l, thus disconnectingthetrip or plunger magnet 24 from the batterylead, so that the switch Ccannot. be operated' while the master switch D is operating and movingthe plungers from one position to another. Furthermore, the motor magnetwhen energized separates the interrupter springs 4l and 42,therebybreaking its own circuit in a manner similar to that of a buzzer.rhe motor magnet 35 continues to operate in this manner as long as thesprings 4G and 47 remain in engagement. the next succeeding tooth on thewheel 48 advances tar enough, the spring 47 disengages from the spring46, thereby opening the energizing circuit :tor the motor magnet. ly thetime this next notch disengages the springs 46 and 47 the masterswitchwiper 37 has passed from the rst contact 3S to the second contact 49.Suppose, however, thatwhen the wiper 37 engages the individual segment49 there is a guarding potential present. In that event an energizingcircuit closed through the motor magnet relay 34 extending' from thecontact through the wiper 37, common segment of the master switch bank,through the winding of the relay 34 to ground G2, as previously traced.'l` he motor magnet relay again energizes and closes the circuit for themotor magnet. The motor magnet operates as before, and the wiper 37 iscarried to the nextcontact, and the next tooth of the wheel 48 separatesthe .springs 46 and 47. This operationcontinues as long as the wiper 37continues to iind contact segments with protecting potentials. Ofceurse. it will be understood that. when the motor uiagnetn thusoperates, the plunger shaft and all plungcrs that are in lockingengagement with the said shaft are moved to a new positonv opposite :inidle bank terminal similar to the one enga led by the line switch C. Atthe instant that the plunger enters the bank the following springs arepressed into Contact: 50 and 5l, 52 and 53, 54 and 55 and 32 and Theengagement of the springs 50 and 5l and 54 I and 55 extends the callingsubscribers line 'conductors 19 and 20 tothe trunk line con- Vductors 56and 57 which terminate in the `rErst selector E. This extension of theline provides an energizing circuit tor the line v relay 58 or the iirstselector. The circuity .line conductor 19', baul trunk conductor-'56,side switch wiper (il,

I'subscribers station,

, carries the spring `iirst selector switch E extends from ground G4through the winding 59, side switclim'iper GO, trunk conductor 57, bankspringsi 'and 54, line conductor 20, transmitter 7 primary Winding 5,iinpulse springs 14land l5,.springs 17 and 1S, .7o

ysprings 50 and 5l.

throughthel winding @2t-o the battery lead v25, thence'througlrbattery Bto ground G.

58l thereupon energiz'es and 75 Thev line relay l operates tocarry-.fthe `spring 63 out ot engagement with thespring 64 and intoengagement with theysp'ring 65, thereby providing an energizing circuitfor the slow acting release ielay;67,fwhich in turn oper- 80 ates toVprovidev vau"energizing circuit for the bridge cut ofl'relay-'G andplunger magnet 24 in series of ythe line switch C. This circuitextendsf'from ground Gr:y through the side switch wiper G5, winding ofthe private trunk lrelay 70, conductor 7l, bank springs 53 and 52,bridge cut off relay 66, trip Ior plunger magnet 24 to the battery lead25, thence through battery' B to ground Gr. The bridge out 01T .relay 90energizes and operates to separate the' springs 2G and-27 and 28 and 29,therebydisconnecting the subscribers line conductors from the lineswitch C. The trip or plunger magnet 24, alsobeifng included in thiscircuit, retains the plunger arm in an attracted position and theplungerlin' the bank terminal. Thus itwill be seen that-the line switch,after being operated from the erated position by a groundconnction atthe first selector switch E. Furthermore, the private line relay 70.alsol energizes and places the springs 72 and 73 in contact, but theiroperation at canoe. The slow-acting release relay 67 also 7 6 from thespring 77 into contact with the spring 7 8.

The foregoing is the .condition of the central oiiice-switches after thecalling sublll),

scriber removes his receiver and before the dial is-turned. When thedial returns to normal position for the iirst digit 2, the is operated.,The iirst'digit being 2, the is twice carried vout of engagement lwiththe impulse spring l5. As a result the energizing circuit through theliner relay 58 is broken twice, and the spring 63 comes into contactwith the spring ('34 twice. After 30 the two impulses are thus deliveredthe line relay 58 again energizes'and carries the spring 63y intocontact with the springb. As previously explained, the` relay 67 is aslowacting relay-that is, slow to de-f1725 energize, and `when thisenergizing circuit is momentarily broken during the transmission of thevertical impulses just delivered. the said relay does not denergize, butremains in an operated position with the springs69'l30 springs G8 and(59, 85

is maintained in its opthis time is of no signiiimpulse spring 14 115 5lt will be seen that from ground G7 through the springs 83 and 84,private magnet 85 to the battery lead 25,

l rotary ..115

and TG in contact with the' springs 68', and,

TS. Then when the spring 63 engages the springL two impulses aretransmitted to thtl vertical magnet 79 overa circuit extending fromground GT through the springs 63 and 64 to the springs 76 and 78,through the private magnet relay 80, through the winding of the vertical11mg-, net TS) to the side switch contactl point 81, through thegsideswitch wiper 82 to the battery lead 25, thence through battery B toground G. The impulses the vertical magnet 79 is thereby energized.twice and the shaft wipers are given two vei'tici'il steps in the wellknown manner. the private magnet relay 80 is included in the energizingcircuit of the verticalinagnet 79. rPhis relay also en ergizes, butbeing a slow-acting-relay it energizes on the tirst impulse and remainsso.

until all the impulses are transmitted. -Y rthis relay, upon energizing,operates to close the springs d3 and 84 in contact, whereby anenergizing circuit is established through the private magnet 85.' Thiscircuit extends thence 'through battery B to ground Gr.4 rlfhel private.magnet relay 80 denergizes after the last vertical impulse, sincethisenergiz ing circuit is nolonger maintained after the springs (53 and (Sedisengage. saidrelay denergizes, permitting the springs' S3- and Se todisengage, whereby the energizing circuit through the private magnet 85is in turn broken, which private magnet now denergizes andpermits-.the'Side switch to pass from first to second position,.permitting the side switch wipers 86 and 82 to engage the contactpoints 87 and 88, respectively. The closure of Contact ,between the sideswitch vwiper 82 and the Contact point 88 sets'up an energizing circuitfor the magnet 89. Thit circuit extends from ground G, througl theinterruptor springs 90,.windin of the rotary magnet 89, contact point88, side switch wiper 82 to the battery lead 25, thence through batteryB and to ground Gr. The said Arotary magnet then opcrates and 99 offtlieselector'E vinto engagement withthe first contact of the' second levelof the selector banks, from which contact it is "assumed the trunk lin'econductors 100, 101 55 and 102 lead to the connector switch F. If thefirst trunk line is busy, however, and the wipers have to pass over busytrunk lines, then' as soon as the private wiper 99 engages tlie rstprivate bank contact .the

private magnet 85V again energizes, locking the sidevswitehvin secondposition. The energizing circuit for said private magnet extends fromthe grounded terminal Gr of.

the battery B through an occupying switch (not shown), through theprivate wiper 99,

' are ycarried beyond the last lbusy trunk being delivered,.

.lay 139` in the Hence, the

Ventered the bankterminals,

l energizing, operates tol rotate. the shaft -wipers 97, 98.

lout lof contact with the thence through the conductor 103 to the sideswitch wiper 80, Contact point 87, through the winding of the privatemagnet S5 to the battery lead 25, thence through battery B to ground Gr.The private magnet 85, upon thusbecoming energized, locks the sideswitch wiper 82 in engagement with the contact point 88, whereby therotary magnet 89 will be energized step. by step untilthe wipers line.At the instant that the lprivate wiper lea-ves the last busy contactpoint the stroyed, and as aresult the selector side switch passes tothird position. If, however, there are no busy trunk lines the privatemagnet 85 releases the side switch to third position asv soon as v.thewipers are.carried into engagementwith the first trunk line. As soon asthe side switch passes to third position, as stated, the trunk lineconductors 50 and 57 4are extendednto the conductors 100 and 101 which,in this case, it is as suined lead to 'the connector switch F. Theextending of the `line occurs, of course, as soon as the side switchwipers 61 and 60 engage the contact points 104C and 105, respectively.1When the trunk line is thus extended te the connector switch F anenergizing circuit is establishedthrough the line resame manner invwhich the energizing circuit was established through the line relay 58of theselector switch E energizing Y circuit through the private magnetis d e-v when the plunger of thc line switch C The energizing circuitthrough the line relay 139 ex tei/ids from ground GrG through thewinding 140, trunk conductors 101.and 57 to the line 'conductor 20,thencethrough the substation to the line conductor 19, trunk conductors50 and 100, thence through the winding 141 to the battery lead 25, andthrough battery B to ground G. The line relay 139, upon from the spring143 to the spring 144. whereby an energizing circuit is establishedthrough the slow acting release relay 115. extending from vground -GS ythrough .the springs 142 and 144, springs 146 and 14T, relay`145 togthe'battery lead 25. thence through battery B to gro-und G. ll'hen therelay 145 energizcs, the spring 14S is shifted spring 149 and intoContact with the spring 150, and the spring 151 is shifted into.engagement with the spring '152.- This primary operation of theconnector relays 139 and 145 occurs at the instant that the side switchwipers (30 and G1 of the selector E pass extend the subscribers lineconductors to the connector F. The engagement of the springs 151 and 152cstablishesa guarding potential for protecting the trunk line conductors100, 101 and 102 against'seiz'ure hy another line and first selector.This potento shift the spi-ing 142 timori tial extends from ground G9 tothe side switch wiper 153, springs 152 and 151, and conductor 102 to theprivate bank contact engaged by the shat't wiper Si). Furthermore, whenthe side switch of the iirst selector E passes to third position a newenergizing circuit is established through the release relay 67, saidcircuit extending from ground G at the connector switch F (Fig. 2),through the springs 152 and 151, conductor 102 to the bank contactengaged by the private wiper 99 of the selector switch E,'side switchwiper SG, contact point 154., conductor 155, relay 67 to batt-ery lead25, thence .through battery B and to ground G.

The operation of the dial for the last two digits aiiects the co torswitch, and when the dial is opera or the second digit 2 the impulsespring i4 is 'twice carried out of engagement with the spring 15,thereby momentarily denergizing the line relay 139 twice, whereby thespring 142 twice engages the spring 143, thus sendin two impulses to thevertical magnet 158 rom ground G8 through the springs 14,2 and 143,springs 14S and 150. private magnet relay 159, conductor 160. throughthe vertical magnet 158, contact point 161, side switch wiper 162 to thebattery lead 25, and through battery B to ground G. The private magnetrelay 159 energizes when the first impulse is delivered, and since thesaidrelay is slow acting it remains in an operated position until allthe vertical impulses are sent. The ven tical magnet, upon beingenergized, operates one step at a time to carry the shaft wipers 163,164 and 165 two steps to a point opposite to the ,second level. Therelay 159, upon energizing` shifts the springs'166 and 167 out ofengagement, and the spring 167 is shifted into engagement with thespring 169. When the springs 167 and 169 engage, an energizing circuitis established through the private magnet 170 from ground G10, throughthe springs 1.67 and 169,- private magnet 170 to the battery lead 25,thence through battery B to ground G. After all the impulses aredelivered, and the line relay 139 returns to its operated position, thecircuit throughv the relay 159becomes permanently broken, thuspermitting the springs I 169;and 167 to return to normal position.

When the springs 169 and 167 disengage, the

circuit through the private magnet 170 be' comesvbrolten, whereby thesaid magnet deenergizes, permitting' the side switch to pass from` firstto second position. The calling subscriber "now operates the callingde.- vice for the last digit thereby transmitting ten impulses andoperating the line Y potential for this trunk relay 139 ten times. Eachtime that the 'line relay springs 142 and 143' engagegzan energizingcircuit is established through the magnet relay 159, conductor 160,springs 156 and 157, through the rotaryv magnet 171 to the contact point172, through the side switch wiper 162 to thebattery lead 25', thencethrough battery B to ground G. Again the private magnet relay energizesas in the previous case, remaining so until all the impulses aretransmitted, thereby retaining the private magnet also energized. Therotary magnet 171 then operates to r0-v tate theshaft wipers intoengagement with the desired line #220. If this line is busy at the time,the connector receives the busy test, whereby the calling subscriber isgiven a busy signal, and it becomes impossible to extend the callingline into connection with the called line. But if the called line is notbusy then, as soon' as the line relay 13u ceases to operate for the lastdigit, the pri'- vate magnet relay 159 denergizes as heretutoreexplained,v thereby permitting the side switch to trip to thirdposition. The result is that the side switch wipers engage, respectively, their contact points of the third position. When the sideswitch wiper 173 engages the Contact point 174 a guarding potential iscompleted at the private wiper 165 for protecting the seized line fromoutside interference while the connection is established. This guardingpotential extends from ground G11 to the contact point 174, thence byway of the side switch wiper 173 to the private wiper 165. Not only doesthis guarding potential prevent other connectors Jt'rom calling in onthe same line over multiples ofthe normal conductors 175 and 176 and177, but it also provides means for 'energizing the cut otl relay 178ot' the line switch C', whereby the switch C is prevented rom operatingwhen the called subscriber removes his receiver 179 from the switch hook180. The trip magnet of the switch C is not operated at this time, asthe resistance of this circuit is such that onlythe cut-oil relay willbe operated, thetrip magnet being so constructed that spond to thisstrength of current, `but requires a stronger current to actuate it. It

will also be seen that when the side switch,`

passes to third position thetrunk release conductor 102 that runsbetween the first selector and the connectorlis provided with guardingpotential from ground G8 through the line relay springs 142 'and 144,back bridge relay springs 146 and 147, contactl lpoint 181, side switchvwiper 153, release it will not rev relay springs 152 and 151 to thetrunk conductor 1 02. However, while. the side switch is fin first andsecond position a guarding ground G0 through the side switch wiper 153direct. The reason forv 4this is `that the guarding potential fromgroundA G8 is interrupted while the line' relay 139 1s operat- 102isprovided fromV some other selector similar to thc selector E stoppingon the same trunl; line. After the side switch y passes tothird'position, however, there is no such possibility, and the potentialis provided trom-ground Grs through the line relay sprin s as stated. Ofcourse, when ihe side switc Wipers 182 and 188 engage their'third-position contact points 184 and 185,

vrelay 190 is energized over the circuit estabside switch wiper 162 tothe battery lead 25, thence ,through battery B to ground G. It Will beseen that as the interru'pter 192 revolves, the energizing circuit otthe ringer relay 190 will be broken periodically, so as not to ring thecalled subscriber continuously. Furthermore, it will be seen that eachtimelt-hat the ringer relay is energized the springs 187 and 189 areseparated Jfrom the ringer` relay springs 1,86 and 188 and brought intocontact with the springs 204 and 205, which latter are the terminals ofthe ringer generator H. As a result ringing current is sent over thecalled line for energizing the bell 206 whilesthe springs 207 and 208are in contact. However, as soon as the called subscriber answers, thebackbridge relay 209 is energized by a iiow ot current from ground G13,winding 210 of the back-bridge relay 209, ringer relay springs 188 and189, side switch wiper 183, contact point 185, line wiper 164, normalconductor 176, conductor 211, transmitter 212, primary winding 218,impulse springs .214,y switch hooi; springs 215 and 208, line conductor217, normal conductor 175, line wiper 168,

Contact point 184, side switch wiper 182,

ringer relay springs 187 and 186, winding 218 of the bacleloridge relay209, thence to the battery lead 25, and through battery B to ground G.Once the back-bridge relay 209 is energized, however, the energizingcircuit or the ringer relay 190 is interrupted at a point between thesprings 193 and 194, thus making it impossible for the ringer relay toenergize as long as the called subscriber is on the line. Furthermore,when the relay 209energizes, the springs 146"and 147 also disengage.thereby interrupting the circuit which provided a guarding potential tothe conductor 102. When thesprings 146 and 147 thus disengage,theresistance coil 221. is included in series instead of in :mul-

tiple as before. rlhe subscribers, being thus connected, may communicateover the inain talking conductors indicated by the l'ieawy lines inFigs'. 1 and 2, and including on one side the condenser 219 at theconnector l", and on the other side the condenser 220.

After the subscribers are through talking, the calling subscriber, if hehangs up his receiver tirst, opens up the line circuit at a pointbetween the springs 17 and 18, thereby initiating the release ot all ofthe switches. This release is brought about as follows: At the instantthat the circuit is broken be tween the said springs 17 and 18 thedoublewound line relay 139 and the release relay 145 of the connector Fdenergize. At the instant that the slow-acting relay 145 and the linerelay 189 denergize, the energizing circuit for the release magnet 222is closed at a point between the springs 148 and 149,`

and at another point between the springs 142 and 148. A flow of currentis, therefore, set up from ground G8 through the springs 142 and 143,through -the springs 148 and 149to the oli" normal spring 223, contactpoint 224 to the release magnet 222, through said magnet to the batterylead 25, thence through battery B to ground Gr. The release magnetinstantly releases the connec-tor switch shaft and restores the sideswitch, thereby breaking the energizing eir- Y cuit through thebaclebridge relay 209 (it the called subscriber has not in the meantiinerestored his receiver), and also breaking the energizing circuit throughthe lnid ;e-cut-oa relay 178 of the called line vswitch C. As soon asthe connector switch shaft reaches its lowest position it breaks thecircuit for the release magnet 222 by separating the-oii1 normal spring223 frogmn the contact point 224, at which instant the release magnet ofthe connector is also restored. The operation of the line relay 189 andof the slow-acting` relay 145 not only causes the release of theconnector, as described, but also the release of the preceding switches,because the ground connection t'roin ground G8 to the trunk-releasecondnctor 102 is interrupted by both of these relays. Since the releaserelay 6T is dependent upon this ground 'for its energized condition, asalready explained, it follows that it will fall back, with the followingresults:

When the release relay 67 of the selector .E

denergizes, the springs 7 6 and 77 are brought into contact, and as aresult the en* ergizing circuit for the release magnet 228 is closed.rllhe energizing current tor this release magnet 228 then flows fromground G7 through the springs and 64, and through the said springs 76and 77 to the release magnet 228, thence through said magnet to thecontact point 229, oli1 normal spring 230 to ,the battery lead 25,thence through battery B te ground Gr. The re- `the iirst selector sideA net 2 becomes broken, and as` a result the bridge-cut-of relay andplunger magnet 24 restore to normal position. Asa result the banksprings 52 and 53, as well as the other bank springs of the lineswitchC, are separated, thus preventing any further action from the trunk.After the switches are thus restored the calling subscriber can, ofcourse, then proceed to make another call.

ln the foregoingit has been pointed out how a subscriber may operatethe. central oiiice switching apparatus vto establish connection with adesired line.- Itwill now be shown how the master switches D and D (Fig.3) control the selection of the individual and common trunks. UponreferringI to Fig. 3 it will be seen that the individual trunks a. ofthe master switch D are allotted to the first contacts of the masterswitch bank level e, and that the individual trunks c of the masterswitch D are allotted to the last contacts of the master switch banklevel c. The remaining contacts in the bank `levels e and e aremultipled and allotted 35 bank levels c and o to the common trunks b.The master switch are so arranged that the master switches may beoperated to carry the plungers of the line switches out of alinementwith the common trunks Z), except when all of the individual trunks arebusy, for then the guarding potential is removed from the bank c or 0,and a common trunk b may be seized. This of contacts corresponding tothe common trunk line contacts in the banks e and e are multipled andprovided with a guarding potential-that is, the last contacts of thebank level o are all multipled and ccnnected with the battery leadthrough the springs 231 and 253'2'. In a similar manner the firstIcontacts ci the bank level c are multipled and connected with thebattery lead through the springs 233 and 234:. Normally, then, thesecontacts are busy, and when the master switch wiper 36a engages any oneof these contacts an energizing circuit is established through the motormagnet relay 34, whereby the motor magnet 35 is in turn energized andoperated to carry the master switch wiper 36a out of engagement with thebusy contacts. In the private line conductor 7l of each individual trunkline there is a relay 7G which energizes and places the springs 72 and73 in contact, when :the "trunk line-,is seiaed,

switches of the arrangementv is as follows z-ln the banks c and c theset' I as. previously explained. When the next trunk is seized the nextrelay energizes andplaces in contact its springs. Thus when all oil thetrunks a are seized, all o't` thevrclays 70 are energized, and anenergizing circuit is established through the relay extending fromground G14 through the series of springs of the relays 70, winding ofthe relay Q35 to the battery B and to ground (i. The relay thereuponenergizes and separates the springs 231 and 232which removes theprotecting potential from the bank level ccntacts corresponding'to thecommon trunks Fg. These trunks are then accessible to line switches ofthe group which is controlled by the master switch D, but not to theline group controlled by the master switch D', since the guardingpotential through the springs 233and 231i is still pres ent on thecornnon trunk line contacts of the bank level c. l/hen all of theindividual trunks a are busy simultaneously the protecting potential forthe common trunks is removed in a similar manner. When all of the trunksa; are busy, and the trunk 236 of the common group b is seized, aprotecting potential is provided at the ybank contact 237, in the usualmanner, to prevent the master switch from stopping here; also, thispotential is transmitted lto the contact 238 of the bank level e of themaster switch D to protect this same trunk line in tbatgroup. As soon,however, as any one 'ot the trunks c, is leftin an idle condition therelay corresponding to the trunk deenergizes, thereby separating itssprings and breaking' the circuit for the relav 235 which removed theprotecting potential from the common trunks The potential is thenestablished, and the common trunks o are not kaccessible until all theindividual trunks a are again busi7 simultaneously. i

Thus it will be seen that with my irnproved arrangement a group of, say`tive trunks may be allotted to one hundred subscribers line switches,'and live more trunks ma;T be allotted to a second group of subscribersline switches; then to these twol groups of subscribers lines may beallotted five trunks in common, ten trunks being'. accessible fromeachsubscribers line switch group. The five trunks which are commontoboth subscribers groups-are notaccessi# ble, however, until all ot' thetive individual trunks are busy.; then a common trunk mayv be seizedwhich is protected by a guarding potential established at bothmasterswitch banks.

From the foregoing it will be seen that I provide an arrangement wherebythe coinmon trunks are only accessible to those subscribers who areunable to find idle trunks in their `own individual groups.;

'These common trunks are all normally provvided with guarding potential,and this guarding potential eirists in the different groups and is onlydestroyed or removed 1n the group or groups where the trunks are allbusy. Thus under no circumstances can a subscriber who has availableforhis use a trunk in his own group obtain connection with one of thecommon trunks. In other words, these common trunks are reserved for whatmay be termed the o-'firloadP-that is to say, the-calls which are madein any group after all of the trunks for that group are busy, and inexcess of what may be regarded as the average maximum number ofcalls-that are liable to occur at the same time. In this way theefficiency-of the coinmon triinks is increased, as they are kept clearof everything except the overload or overflow of calls from thedifferent groups, and are never available for any other purpose-that istosay, can never be used by calling subscribers in groups where thereare still idle trunks to be found.

"While I have shown a systemiii wliich'all the switches are located at asingle central oiice, it is obvious that, if desired, some of theseswitches, suoliv as the line switches, for example, could be located ata subcentral station, which station would be connected -with the centralstation by means of the groups of individual and multiple trunks.`

vWhat I claim as my invention is z*- 1. In a telephone exchangeltrunking systeni2 a plurality of groups of subscribers stations, a Setof trunks for each group, another set of trunks common to all ,of thegroups, means normally precluding use of said common trunks, and meansserving to automatically render said common trunks available forservice'whenever all of the trunks of any group are in use.

2. In a telephone exchange trunking systein, a subcentral station.havin@ the substations thereof divided into a ptlurality of groups, asmany sets of trunks leading to said subcentral station as there'aregroups, another set of trunks leading to said subceiitral station, saidlast set being common to all of the groups, means normally precludinguse of said common trunks, and means serving to automatically rendersaid com mon trunks available for service whenever all of the trunks ofany 'group are in use.

In a ,telephone exc iange trunking sys-- teni, subscribers stationsdivided into a plurality of groups, a trunking switch at the centralstation toi' each subscriber, a set of n n I l trunks .terminating onlyin the switches ot one group, another set of trunks terminat- .ing onlyin the switches of another group, a

third set of trunks terminating in the switches-oic all of the groups,means normally precluding use of said common trunks, and means servingto automatically render :said common trunks available :for serviceraider/i Whenever all of the trunks of any group are in use. i

Il. In a telephone exchange trunking system, subscribers stationsdivided groups, a set of five trunks for one group, a set ot" fivetrunks for another group, a set of five trunks common to all of thegroups, whereby each subscriber has access to any.

one of ten trunks, live of which are individual to one group and five ofwhich are common to all of the groups, means normally precluding use ofsaid common trunks, and means serving to automatically render saidcommon trunks available for service whenever all of thetrunks ofanygroupare in use. v

5. In a telephone exchange trunking system, ak group of subscribersstations, an-

other group of subscribers stations,I a set of trunks for one group, aset of trunks for the other group, a third set of trunks common to bothgroups,'a master switch and trunking switches controlledy thereby foruse by the subscribers of one group, a master switch and trunkingswitches controlled thereby for use by the subscribers of the othergroup, whereby the trunks that are common to the two groups are subjectto automatic selec tion by more than one master switch, each masterswitch operating independently of the 0ther,'means vnormally precludinguse of said common trunks, and means serving to automatically rendersaid common trunks available for service whenever all of the trunks ofany `group are in use.

6. In an automatic system in which calls are made and released over thetwo sides of the line circuit in series, a group of subscribers9stations,l .anqth'er group of subscribers` stations, a set of trunks forvone group, a set of trunks for the other group, a third set of trunkscommon to both groups, amaster switch and trunking switches controlledthereby for use by the subscribers of one groupand a master switch andtrunking switches controlled thereby for use by the subscribers of theother group, whereby the trunks that are common to the two groups aresubject to automatic selection by more than one master switch, eachmaster switch operating independently of the other, said system beingoperative for calling and releasing independently of the ground at thesubstations thereof.

7. In a telephone exchange trunking system. a plurality of groups ofsubscribers stations, a set of trunks for each group, another set oftrunks common to all of the groups, means normally establishing aguarding potential for all common trunks, a relay for each individualtrunk, and a relay for each group, adapted teV remove said guardingpotential when all of the trunk relays of the group are energized.

8. In a telephone exchange trunking sys into tem, a subcentral stationhavin the substations thereof divided into a p urality of groups, asmany sets of trunks leading to said subcentral station as there aregroups, another set of trunks leading to said subcentral station, saidlast set being common to all of the groups, means normally establishinga guarding potential for all common trunks, a relay for each individualtrunk, and a relay for each group, adapted to remove said guardingpotential when all of the trunk relays oi the group are energized.

` 9. In a telephone exchange trunking system, subscribers stationsdivided into a plurality of groups, a trunking switch Vat the centralstation for each subscriber, a set of trunks terminating only in theswitches of one group, another set of trunks terminating only in theswitches of another group, a third set of trunks terminating in theswitches'of all of the groups, means normally establishing a` guardingpotential for vidual trunk, and a relay for each group, adapted toremove said guarding potential when all of the trunk relays of the groupare energized.

10. In a telephone exchange trunking system, subscribersstations divided-into groups, a set of five'trunks for one group, a set of ve trunks foranother group, a set of ve trunks common to all'of the groups, wherebyeach' subscriber has access to any one of ten trunks, five of which areindividual to one group and iive of whichare common tc all of thegroups, means normally establishinga guarding potential for all commontrunks, a relay for each individ-V ual trunk, v and a relay for eachgroup, adapted to remove l said guarding potential when all of the trunkrelays of the group are energized. Y v

' pendently 11, In a telephone enchange trunking system, a group ofsubscribers stations, another group of'subscribers stations, a set 'oftrunks for one group, a set of trunks for the other-group, a third -setof trunks common to both groups, a master switch and trunking switchescontrolled thereby or use by the subscribers ot one group, a masterswitch and trunking switches controlled thereby for use by thesubscribers of the other group, whereby the trunks that are common tothe two groups are subject to automatic selection by more than onemaster switch, leach master switch operating -indeof the other, meansnormall establishing a guarding potential for al common trunks, a relayfor each individual trunk, and a relay for each group, adapted to removesaid 'guarding potential when Iall of thetrunk relays of the group areenergized.`

'12. In a' telephone'isystem, a trunk, an-

.wherebyl said guarding potential when the rst trunkl is in use.,

18. In a telephone system, a set of trunks, a reserve trunk, andautomatic means the reserve trunk will. test busy until a the othertrunks are in use.

14. In a telephone system, trunks arranged in groups, suiicient innumber to carry the average normal load, a reserve set of trunks forcarrying the overload, common to the diierent groups, and automaticmeans preventing use .of the reserve trunks except when the trunks ofone or more groups are all busy.

15. In a telephone system, a trunk, means serving normally to maintain aguarding potential for said trunk to prevent use- 'remove said guardingpotential when said y' trunk is to be used. all common trunks, a relayfor each indi- 16. In a telephone system, a trunk, means whereby thesaid trunk will normally test busy to prevent use thereof, and meansfor' automatically changing the test to show the tial when all ,of thetrunk relays are en-l ergized.4

18. In a telephone system, ,trunks arsoA ranged in groups, a set or'trunks common to the said groups, a group of substations for each ofsaid groups of trunks,and automatic means whereby the sa d common trunksare available for use only y calling substations of en group or groupsin which the trunks are a lbusy. 3,

19. In a telephone system, a plurality of master switches, a group oftrunks for each master switch, a group of trunks common to said masterswitches, means for normally maintaining a guarding potential for saidcommon trunks in all of said switches, anddevices whereby the guardingpotential is removed in one switch, when all of the individual trunksthereof are busy, without removing the-guarding potential in the otherswitchor switches.

20. In a telephone system, a group of'automatic switches, a roup oftrunks common to all of said switc es, .a group of trunks individual toeach ofsa'id ou sof switches, and means including ay p ura ity of relaysdi of said individual trunks; me

connected in seres -'oxjmantanng a @marng 'poenzd on said common gmu;'VE runks until a the runs of an indiv/1d uff. group are in use.

En. a telephone sys'fem, a group of wf 'inomafc Switches, :l group oftrunks common sal G23 sai switches., a, group of unks :llvdmto one ofsaid groups; of sw''tchas, m 'means including a remy mm2 n swiss withauch of said ndvdua g-oup of Hunks im mainaning a gaaing paen- 'el on.said cmmongroup S 'l use'.

23. in s@ telephone sysem, a, pumity' off' gmups 'subsrbers ineS, absefs 0i mmks o each gToup, amther set of trufuks common 'o of smidgiups, mon-mme'fc. swzcha associated with the subscribers$ Eines "forestablishing Connecou Wh said trunks, means'normny precudng use of said,common trunks, @n means serving to uomuticay rencer Esd 'common t-unks:

\ avauba for service whenever all the trunks nf any group are in use.

Sgmef 'by m@ M, Chc'agoa 390k Cammy, lwis, ibi, 3 mf 511118, 1909;BEWARE A. MELMPGEE.

"Witnessw: EDWARD "LL FALLS? ARTHUR l RAY.

